1. Field of the Invention
This invention relates to an image forming apparatus, such as a copier, a printer, a facsimile, etc., and a multifunctional machine including these apparatuses. In particular, this invention relates to a color image forming apparatus including a plurality of stations arranged side by side for forming and transferring different component color toner images from a plurality of image bearing members to a transfer medium in turn, either directly or indirectly via an intermediate transfer member.
2. Description of the Background Art
Recent office instruments have become required to handle a large number of color documents. As a result, a compact and high-speed color image forming apparatus, such as a full-color printer, a full-color copier, etc., is being demanded. In a recently commonly used color laser printer, a one-drum type is becoming main stream. Specifically, in such a device a plurality of developing devices are commonly used and accessibly arranged in the vicinity of an image-bearing member, so that respective color toner images are formed per its rotation and transferred from the image-bearing member to the transfer medium to form a full color toner image thereon.
The one drum type image forming apparatus is classified into two categories. One of them is an intermediate transfer system, in which a plurality of color toner images are formed on an image bearing member and primarily transferred and superimposed onto an intermediate transfer member. The superimposed color toner images are then secondarily transferred onto a transfer medium. The other type is a direct transfer system, in which a transfer medium is carried and conveyed by a transfer drum and a conveyance belt or the like. A plurality of color toner images are formed on image bearing members and are transferred onto a transfer medium in turn, and thereby a full color toner image is formed thereon.
A tandem (or an inline) type image forming apparatus has been proposed to realize speeding up of a color image formation. Specifically, three or four image bearing members are arranged side by side, and accordingly, a plurality of stations are arranged. A plurality of latent images are formed on the image bearing members at the respective stations and are developed by respective developing apparatuses. Then, the toner images are transferred in turn and superimposed on a transfer medium, and thereby a full color image is formed. For example, JOP53-74037 proposes an image forming apparatus with a plurality of image bearing members capable of transferring in turn and superimposing toner images on a transfer medium, which is conveyed by a belt type conveyance device.
Such a tandem type image forming apparatus allows faster image formation than that of the above-mentioned one drum type image forming apparatus by four times if peripheral speeds of their image bearing members are the same.
JOP59-192159 proposes an image forming apparatus that employs a tandem system with an intermediate transfer member.
Further, JOP2000-242152 proposes an image forming apparatus employing a collecting device that collects toner, which is transferred from the upstream image bearing member to the intermediate transfer belt, and which can be possibly transferred back to a downstream image bearing member due to its own reverse polarity. That is, a part of toner carries a polarity opposite to a normal polarity when transferred from the upstream image-bearing member to the intermediate transfer member and can be transferred back to the image-bearing member when downstream side color toner is transferred.
JOP10-293432 discusses a method of removing background potential before transferring a toner image to prevent toner from being oppositely charged, i.e., from transferring back.
JOP09-106136 proposes a multicolor image forming apparatus that uses magnetic toner of one and two component developer in first and second developing processes, respectively. A magnet is arranged inside of a photoconductive belt serving as an image-bearing member while being opposed to a developing sleeve arranged in a second developing device to increase an adhering force of the one component magnetic toner to the photo-conductive belt.
JOP2000-235291 discusses a technology in which a prescribed amount of charge is applied to toner of a toner image of a first color. JOP2001-209232 discusses a technology in which either a transfer process or a developing process is controlled to suppress an electrostatic attracting force applied to a latent image-bearing member that carries toner of reverse polarity. JOP2001-265087 discusses a technology in which a toner ratio of later used developer is more increased than that of previously used developer. Further, JOP2001-34045 proposes a method of decreasing a toner ratio of later used developer more than that of previously used developer while adjusting a direct current component of a developing bias.